The present invention relates to an optical information reproducing apparatus suitable for use in such equipment as an optical video disk player or a digital audio disk player.
A conventional optical information reproducing apparatus is shown in FIG. 1A. A light beam issuing from a light source 1 such as a laser diode is reflected from a beam splitter 2 and converged by an objective lens 3 to form a small spot of information detecting light on the recording surface of a disk 4. The reflected light from the recording surface of the disk 4 becomes an optical signal carrying the information recorded as a recess (pit) in the disk 4. This optical signal passes through the objective lens 3 and beam splitter 2 in succession to a photodetector 5, where the optical signal is converted to an electrical signal to reproduce the recorded information. The objective lens 3 is driven with an actuator 6 in two mutually perpendicular directions, one being parallel to the recording surface of the disk 4, and the other being perpendicular to that surface. The lens 3 is controlled such that its focal point will coincide with the recording surface of the disk 4, whereas the spot of information detection light will be positioned on a track on the recording surface of the disk 4. In the system of FIG. 1A, the optical unit is functionally equivalent to a non-cofocal optical unit as shown in FIG. 1B.
As described by H. H. Hopkins in his article entitled "Diffraction Theory of Laser Read-out System for Optical Video Disk" (J.0.S.A., Vol. 69, No. 1, January, 1979), the conventional apparatus shown in FIG. 1A has incoherent transmission characteristics with respect to the spatial frequency of the information recorded in the disk, and has suffered from the disadvantage that the level of a signal detected becomes low at a high spatial frequency, as illustrated in FIG. 2. Further, no output signal is detected at higher spatial frequency so that it is impossible to detect the information recorded in the disk.